As shown, for example, in a schematic perspective view in FIG. 1, a fuel cell separator 10 is formed by providing a plurality of partition walls 12 in a protruding state at predetermined intervals on both sides of a fiat plate portion 11. In order to form a fuel cell, a number of fuel cell separators 10 are laminated along the protruding direction (the vertical direction in the figure) of the partition walls 12. By this lamination, a constitution is realized, wherein reactive gas (hydrogen or oxygen) is allowed to flow through channels 13 formed by pairs of adjacent partition walls 12. The fuel cell separator is produced by molding a resin composition containing a resin material and a conductive material such as graphite into the above-mentioned shape.
As a method for forming the fuel cell separator, there is generally used a method in which the composition containing a thermosetting resin such as a phenol resin or an epoxy resin as the resin material is placed in a mold provided with flow paths for gas or cooling water, and molded by heat compression molding in which the composition is hot pressed. However, in recent years, in order to improve productivity, it has been tried to produce the fuel cell separator by injection molding, instead of heat compression molding. For example, there is known a method of injecting a resin composition containing a graphite material and a thermoplastic or thermosetting resin from a cylinder into a mold, thereby forming a fuel cell separator (see Patent Documents 1 to 3). In such injection molding, the resin composition is transferred to the closed mold through a narrow passage called a runner. When the resin composition has low fluidity, a short shot occurs in which a part of the mold can not be filled or high pressure is required for filling, so that the inner pressure of the mold increases to cause deformation of the mold, leading to deterioration of dimensional accuracy of a molded article, in some cases. Accordingly, in order to fill the mold without clearance with the resin composition to obtain a molded article having high dimensional accuracy, the resin composition is required to have high fluidity.
On the other hand, an epoxy resin has been widely used as a resin material. However, a curing agent and a curing accelerator are necessary to cure the epoxy resin, and an organic phosphine such as triphenylphosphine has been generally used as the curing accelerator (see Patent Document 4). However, only a fuel cell separator having low conductivity is obtained from the resin composition using the organic phosphine as the curing accelerator. In particular, when artificial graphite is used as the carbon material and triphenylphosphine as the curing accelerator is used in combination therewith, conductivity deteriorates. Moreover, when natural graphite is used as the carbon material, impurities of metal components are contained in large amounts, which sometimes adversely affect an electrolyte membrane used in a fuel cell.
Consequently, it has been tried to use a urea compound as the curing accelerator (see Patent Documents 5 and 6). However, with regard to the resin composition described in Patent Document 5, fluidity as a material is poor and the composition cannot be molded by injection molding in the tracing experiments done by the present inventors. Moreover, in the resin composition described in Patent Document 6, expanded graphite is used as the carbon material, and impurities derived therefrom are contained in large amounts, which possibly adversely affect the electrolyte membrane.
As described above, a fuel cell separator resin composition excellent in conductivity and fluidity and containing little impurities has not been obtained by the conventional technique.